In the disclosure of the present invention reference is mostly made to the treatment of diabetes, however, this is only an exemplary use of the present invention.
The most common type of durable drug delivery devices adapted to receive a drug filled cartridge and expel a set dose therefrom are driven by manual means or by a spring energized during dose setting, the cartridge being of the type comprising an axially displaceable piston having an initial proximal position. The device may be pen-formed or in the form of a more box-shaped so-called doser. In order to improve convenience, user-friendliness and provide additional features, e.g. detection and storing of expelling data, drug delivery devices have been provided with electrically driven means, typically in the form of an electronically controlled motor driving a piston rod through a gear arrangement, e.g. as shown in U.S. Pat. No. 6,514,230 and US 2011/306927. The same arrangement is also used in infusion pumps, e.g. as shown in U.S. Pat. No. 7,193,521. When a new cartridge is loaded into a motor-driven drug delivery device the piston is first moved proximally allowing a full cartridge to be inserted after which the piston rod is moved into contact with the cartridge piston to bring the drug delivery device in an operational state. The loading of a new cartridge may be performed manually, i.e. the user actuates buttons moving the piston rod back and forth, or it may be partly or fully automated, e.g. the device detects that a cartridge cover is opened and moves back the piston rod, this allowing the user to remove the used cartridge, insert a new and close the cover. When the device detects that the cover has been closed and a cartridge is inserted the piston rod is automatically advanced into engagement with the piston where after the device is ready for use or further initial operations, e.g. a user operated air shot to drive out air in the cartridge or an attached needle.
As the piston in a new cartridge due to manufacturing tolerances is not located at the exact same position, just as a user may also insert a partly used cartridge, the piston rod cannot be automatically advanced to a fixed position but have to be forwarded until engagement with the piston is detected by the motor controller. In the past it has been proposed to detect piston engagement by directly or indirectly detect pressure-build up in the system. For example, when the piston rod engages the piston the resistance to movement will increase and the electric current to the motor will in many control systems increase which can then be detected and used to determine piston engagement, however, this method has a number of drawbacks. As the motor does not stop immediately the cartridge will be pressurized which will subsequently result in an undesired flow of drug when a needle is connected to the cartridge. This can be somewhat compensated by increasing the sensitivity of the current sensing system, but the more sensitive the higher the risk that a false detection occurs because of the variation in the motor current caused by variation in mechanical friction in the motor/gear components. WO 01/37905 discloses an arrangement using a light source and a light sensor arranged on a piston driver to detect when the piston driver has been forwarded into contact with a piston, this resulting in a delay when stopping the motor. Undesired pressurization may also be mitigated by moving back the piston rod slightly, however, it may either be moved too little or too much backwards. A further problem arises when a needle has been connected before the piston rod is advanced as this reduces the pressure build, often to an extent that piston engagement cannot be detected due to a low signal-to-noise ratio. Whether or not engagement is detected with a delay or not at all, the result is an undesired flow of drug. Alternative pressure build up in the system could be detected by incorporating a dedicated sensor, e.g. a strain gauge attached to a component which will be deformed, however, such a system will typically also suffer from a delay in sensing.
Having regard to the above, it is an object of the present invention to provide a motorized drug delivery device adapted to receive a drug-filled cartridge, and in which a piston drive member can be moved automatically into engagement with a cartridge piston in a simple and convenient way, the arrangement being user-friendly, cost-effective as well as reliable. Engagement should be achieved with only a minimum of pressure build-up.